1. Field of the Invention
The present invention relates generally to an automotive vehicle steering system and more specifically to a power assisted type power steering system wherein rotational energy is added directly to a rotatable steering shaft of the system.
2. Description of the Prior Art
FIGS. 1 and 2 show a prior art arrangement (disclosed in Japanese Patent Application First Provisional Publication No. 47-20835) wherein, in order to provide power assistance to the steering and thus reduce the physical force which must be manually applied by the driver, the steering shaft is formed in first and second sections 2, 3 which are interconnected by a differential gear 4. A motor (not shown) is connected to the differential gear 4 and arranged to selectively energizable to add rotational energy to the output section 3 of the divided steering shaft.
However, this arrangement has encountered the drawback that it is necessary to provide a reduction gear such as a worm gear arrangement between the motor and the element of the differential gear via which the auxiliary power input is achieved. Viz., in the illustrated arrangement the differential gear 4 takes the form of a planetary gear train wherein the sun gear A is connected to the input section 2 of the steering shaft, the pinion carrier B is connected to the output section 3 of the steering shaft and the ring gear C is connected with the gear 5 driven by worm D. For the sake of discussion let it be assumed that the rotational speeds of the input and output sections 2, 3 of steering shaft are .omega..sub.I and .omega..sub.o while the rotational speed of the ring gear C is .omega..sub.c, and that the pitch circle diameters of the of the sun gear A and ring gear C are `a` and `c` respectively; then it can be shown that in the case of the FIG. 1 arrangement: ##EQU1## while in the case of the FIG. 2 arrangement that: ##EQU2##
Accordingly, it can be seen that the rotational speed .omega..sub.o of the output section 3 is equal to the sum of .omega..sub.I and .omega..sub.c. Viz., the steering ratio is varied according to the .omega..sub.c input. However, with the FIG. 1 arrangement as the planetary gear must be arranged so that `c` is greater than `a` then equation 1 becomes ##EQU3##
This renders the system impractible unless the variation in steering ratio is rendered stable by rendering .omega..sub.c low in value. In order to achieve this the above mentioned worm gear arrangement is utilized. The worm gear D is also useful in that it functions as a brake which renders the ring gear C stationary when the motor is not energized.
However, the provision of this gearing (5, D) causes the system to become heavy and bulky thus causing design difficulties when trying to incorporate such a system into the confines of an automative vehicle.
Further, when the value of .omega..sub.c is zero (viz the motor is off) the ratio of .omega..sub.I /.omega..sub.o is given by a/a+c and since c is greater than a then it can be shown that a/a+c&gt;&gt;1. Accordingly, in order to allow for the situation wherein the motor stops as in the event of a malfunction, it has been necessary to provide a specially designed steering gear between the output section of the steering shaft and the vehicle wheels. This use of a non-standard gearing of course increases the cost of the device.
On the other hand, in the case of the FIG. 2 device, as will be appreciated from equation 2 in which the coefficient of the control rotation speed .omega..sub.c is -a/c, stable steering ration variation can be attained by increasing the diameter of the ring gear, viz., increasing `c`. However, the ratio of a/c is limited in actual fact to a value of 0.3 and thus this arrangement tend to be subject to a deteriorated steering `feel` due to the increased mass of the ring gear C. This arrangement requires increased amount of space and again induces the need to use the above mentioned worm gear D.
Further, as the ratio .omega..sub.I /.omega..sub.o between the steering input and output is ##EQU4## it has been necessary in this instance to use a non-standard steering gear also.